Electrostatic discharge (ESD) is typically a high-voltage short-duration event, typically involving voltages in the range of hundreds to thousands of volts, and a typical duration of 10 to 150 nanoseconds. Devices such as electronic components, electronic circuits and systems, and larger systems such as automobiles, are commonly tested to confirm that they can withstand various standardized ESD events. For ESD testing, one common way to emulate ESD in a standard manner is to charge a floating transmission line to a pre-determined high voltage, and then discharge the transmission line into a Device Under Test (DUT). This method is called Transmission Line Pulsing (TLP). FIG. 1A illustrates a basic TLP test system 100. A high voltage DC supply 102 charges a transmission line 106 through a high impedance (Rs) and a switch 104. After the transmission line 106 is charged, the switch 104 connects one end of the transmission line to a DUT 108. The transmission line 106 generates a single voltage pulse with a pulse width determined by the propagation time along the length of the transmission line. A termination impedance RL matches the impedance of the transmission line 106 to prevent reflections.
Some ESD events may be more complex than a single voltage pulse. For example, they may involve a positive pulse followed by a negative pulse. In FIG. 1A, if the termination resistance RL is eliminated (effectively infinite termination resistance), then a reflected pulse will be generated having an opposite polarity from the high voltage DC power supply. Alternatively, the termination resistance may be made a different impedance than the impedance of the transmission line and the voltage waveform will be partially reflected at the impedance discontinuity.
In some test systems, the goal of ESD testing is to confirm that the DUT can withstand a standard emulated ESD voltage waveform. In other test systems, the goal is to characterize the impedance of a device or circuit that is intended to protect a system during ESD. A protective device (for example, a silicon controlled rectifier or thyristor) may turn on (called snapback) at a high voltage, and then conduct current at a reduced voltage (called the holding point). The voltage waveforms needed to characterize the impedance of a protective device at multiple current levels need to have a peak voltage sufficient to turn on the protective device and then “stair step” down to one or more holding point voltages. The resulting test system is called a multilevel TLP system. FIG. 1B illustrates an example multilevel TLP test system 110. In FIG. 1B, there are two transmission line sections (114, 116) of different lengths, in series, separated by a series resistor RSERIES. Both sections are pre-charged. When the switch 118 is closed, a pulse is generated in each section of the transmission line. The voltages are partially reflected in each section of the transmission line at each impedance discontinuity. The pulses and reflections add and cancel at the DUT to form a “stair-stepped” multi-level voltage pulse. There are many variations, with varying placements of the DUT, switches, and resistances.
There are many different standards for ESD test systems for different product categories. There are standard systems that emulate ESD originating from a person (Human Body Model), standard systems for the automotive industry (ISO 10605, Zwickau), standard systems for electronic products such as cell phones, computers and televisions (IEC 61000-4-2), and standard systems specified by other standards bodies such as JEDEC, ESDA, and JIETA. Some ESD tests need complicated setups that are typically done only by the manufacturers of a final product, such as an automobile. This is a problem for vendors supplying parts or subsystems for multiple final product categories. The parts and subsystems vendors need to be able run a wide variety of different standard tests. There is a need for an ESD test system that can flexibly produce a wide variety of voltage waveforms consistent with a wide variety of industry ESD test standards.